The present invention relates to bone fixation devices and, more particularly, absorbable bone fixation pins of the type for securing two or more adjacent bone fragments together to provide shear-force stabilization during the healing period.
Bones which have been fractured, either by accident or severed by surgical procedure, must be kept together for lengthy periods of time in order to permit the recalcification and bonding of the severed parts. Accordingly, adjoining parts of a severed or fractured bone are typically clamped together or attached to one another by means of a pin or a screw driven through the rejoined parts. Movement of the pertinent part of the body may then be kept at a minimum, such as by application of a cast, brace, splint, or other conventional technique, in order to promote healing and avoid mechanical stresses that may cause the bone parts to separate during bodily activity.
The surgical procedure of attaching two or more parts of a bone with a pin-like device requires an incision into the tissue surrounding the bone and the drilling of a hole through the bone parts to be joined. Due to the significant variation in bone size, configuration, and load requirements, a wide variety of bone fixation devices have been developed in the prior art. In general, the current standard of care relies upon a variety of metal wires, screws, and clamps to stabilize the bone fragments during the healing process. Following a sufficient bone healing period of time, the percutaneous access site or other site must often be reopened to permit removal of the bone fixation device.
Long bone fractures are among the most common encountered in the human skeleton. Many of these fractures and those of other small bones and small bone fragments must be treated by internal and external fixation methods in order to achieve good anatomical position, early mobilization, and early and complete rehabilitation of the injured patient.
The internal fixation techniques commonly followed today frequently rely upon the use of Kirschner wires (K-wires), intramedullary pins, wiring, plates, screws, and combinations of the foregoing. The particular device or combination of devices is selected to achieve the best anatomic and functional condition of the traumatized bone with the simplest operative procedure and with a minimal use of foreign-implanted stabilizing material. A variety of alternate bone fixation devices is also known in the art, such as, for example, those disclosed in U.S. Pat. No. 4,688,561 to Reese, U.S. Pat. No. 4,790,304 to Rosenberg, and U.S. Pat. No. 5,370,646 to Reese, et al.
Notwithstanding the common use of the K-wire to achieve shear-force stabilization of bone fractures, K-wire fixation is attended by certain known risks. For example, a second surgical procedure is required to remove the device after healing is complete. Removal is recommended, because otherwise the bone under an implant becomes vulnerable to stress shielding as a result of the differences in the modulus of elasticity between metal and the bone.
In addition, a permanently implanted K-wire may provide a site for a variety of complications ranging from pin-tract infections to abscesses, resistant osteomyelitis, septic arthritis, and infected nonunion.
Another potential complication involving the use of K-wires is in vivo migration. Axial migration of K-wires has been reported to range from 0 mm to 20 mm, which can both increase the difficulty of pin removal as well as inflict trauma to adjacent tissue.
As conventionally utilized for bone injuries of the hand and foot, K-wires project through the sldn. In addition to the undesirable appearance, percutaneously extending K-wires can be disrupted or cause damage to adjacent structures such as tendons if the K-wire comes into contact with external objects.
Notwithstanding the variety of bone fasteners that have been developed in the prior art, there remains a need for a bone fastener of the type that can accomplish shear-force stabilization with minimal trauma to the surrounding tissue both during installation and following bone healing.
There is provided in accordance with one aspect of the present invention a bone fixation device of the type useful for connecting two or more bone segments during the healing process. The device comprises an elongate pin body having proximal and distal ends, and a distal anchor on the distal end of the pin body. A proximal anchor is axially movably carried on the pin body.
An anchor retention structure on the pin body permits distal motion of the proximal anchor with respect to the pin body, but resists proximal motion of the proximal anchor with respect to the pin body. In one embodiment the retention structure comprises at least one ramped surface that inclines radially outwardly in the distal direction. Preferably, the bone fixation device comprises a bioabsorbable material, such as poly-p-dioxanone, poly-L-lactide or other materials.
In accordance with a further aspect of the present invention, there is provided a method of fixing two or more adjacent bone components with respect to each other. The method comprises the steps of drilling a hole through a first and a second adjacent bone component. A bone fixation device of the type having an elongate pin body, a distal anchor on a distal end of the pin body, and a proximal anchor axially movably supported by the pin body is provided. The bone fixation device is advanced distally through the hole until the distal anchor projects through the distal end of the hole. A proximal traction is applied to the pin body, while the proximal anchor is advanced distally along the pin body to apply a compressive force to the bone components. The compressive force is retained by resisting proximal movement of the proximal anchor with respect to the pin body. Preferably, the fixation device comprises a bioabsorbable material such as poly-p-dioxanone, so that the fixation device will be absorbed by the body, thereby eliminating the need for a post-healing removal step. Certain nonferrous materials may also have the advantage of sharing loads through healing bone, as compared to metals which spare loads and conduct energy much differently than the bone they are invested in. Load sharing may improve healing by allowing more normal bone physiology to occur around a fracture or osteotomy site.
Further features and advantages of the present invention will become apparent to those of skill in the art in view of the detailed description of preferred embodiments which follows, when considered together with the attached claims and drawings.